Converting to Smart Object

If you open the smart object, you can copy the mask just like you can copy any other image element. Close and save, and paste into a new mask in the master document.

You'll also need to delete the smart object mask once you have a master document mask.

NOTE that when I just tried this, the smart object cropped/trimmed transparent pixels, IOW those parts that were masked out. There was no way to retrieve the whole image area from the smart object. This seems like a bug, but I don't have time to look into it now.

You can sort of undo a Smart Object.

Double click to open it in its own window

Select all the layers.

Right click and choose Duplicate, and chose the original document.

That gets you the constituent layers back to do with as you please.

Or you can open the Smart Object and Ctrl click the layer mask to load it as a selection.

Go back to the master document, and add a layer mask to the Smart Object, or Group that contains it.

The layer mask will be the same as the original, but will have limited function because you would only be able to conceal, and not reveal.

Without seeing exactly what your issue is, we can only guess, but my best guess is to break out the SO layers using duplicate.

Good luck

The layer mask has been applied to the object.  You can if you wish mask the smart object layer.  You can also save the layer mask as an alpha channel before converting the layer to a smart object layer.  The layer mask also does not disappear it is part of the object. you can open the object in Photoshop and edit the layer mask.